Isolation region forming methods

ABSTRACT

In one aspect, the invention includes an isolation region forming method comprising: a) forming an oxide layer over a substrate; b) forming a nitride layer over the oxide layer, the nitride layer and oxide layer having a pattern of openings extending therethrough to expose portions of the underlying substrate; c) etching the exposed portions of the underlying substrate to form openings extending into the substrate; d) after etching the exposed portions of the underlying substrate, removing portions of the nitride layer while leaving some of the nitride layer remaining over the substrate; and e) after removing portions of the nitride layer, forming oxide within the openings in the substrate, the oxide within the openings forming at least portions of isolation regions. In another aspect, the invention includes an isolation region forming method comprising: a) forming a silicon nitride layer over a substrate; b) forming a masking layer over the silicon nitride layer; c) forming a pattern of openings extending through the masking layer to the silicon nitride layer; d) extending the openings through the silicon nitride layer to the underlying substrate, the silicon nitride layer having edge regions proximate the openings and having a central region between the edge regions; e) extending the openings into the underlying substrate; f) after extending the openings into the underlying substrate, reducing a thickness of the silicon nitride layer at the edge regions to thin the edge regions relative to the central region; and g) forming oxide within the openings.

RELATED PATENT DATA

This patent resulted from a divisional application of U.S. patentapplication Ser. No. 09/146,838, which was filed on Sep. 3, 1998.

TECHNICAL FIELD

The invention pertains to methods of forming isolation regions and canhave particular application to methods of forming shallow trenchisolation regions.

BACKGROUND OF THE INVENTION

In modern semiconductor device applications, numerous individual devicesare packed onto a single small area of a semiconductor substrate. Manyof these individuals devices need to be electrically isolated from oneanother. One method of accomplishing such isolation is to form atrenched isolation region between adjacent devices. Such trenchedisolation region will generally comprise a trench or cavity formedwithin the substrate and filled with an insulative material, such as,for example, silicon dioxide. Trench isolation regions are commonlydivided into three categories: shallow trenches (trenches less thanabout one micron deep); moderate depth trenches (trenches of about oneto about three microns deep); and deep trenches (trenches greater thanabout three microns deep).

Prior art methods for forming trench structures are described withreference to FIGS. 1-12. Referring to FIG. 1, a semiconductor waferfragment 10 is shown at a preliminary stage of a prior art processingsequence. Wafer fragment 10 comprises a semiconductive material 12 uponwhich is formed a layer of oxide 14, a layer of nitride 16, and apatterned layer of photoresist 18. Semiconductive material 12 commonlycomprises monocrystalline silicon which is lightly doped with aconductivity-enhancing dopant. To aid in interpretation of the claimsthat follow, the term “semiconductive substrate” is defined to mean anyconstruction comprising semiconductive material, including, but notlimited to, bulk semiconductive materials such as a semiconductive wafer(either alone or in assemblies comprising other materials thereon), andsemiconductive material layers (either alone or in assemblies comprisingother materials). The term “substrate” refers to any supportingstructure, including, but not limited to, the semiconductive substratesdescribed above.

Oxide layer 14 typically comprises silicon dioxide, and nitride layer 16typically comprises silicon nitride. Nitride layer 16 is generally fromabout 400 Angstroms thick to about 920 Angstroms thick.

Referring to FIG. 2, patterned photoresist layer 18 is used as a maskfor an etching process. The etch is typically conducted utilizing dryplasma conditions and CH₂F₂/CF₄ chemistry. Such etching effectivelyetches both silicon nitride layer 16 and pad oxide layer 14 to formopenings 20 extending therethrough. Openings 20 comprise peripheriesdefined by nitride sidewalls 17 and oxide sidewalls 15. The etchingstops upon reaching silicon substrate 12.

Referring to FIG. 3, a second etch is conducted to extend openings 20into silicon substrate 12. The second etch is commonly referred to as a“trench initiation etch.” The trench initiation etch is typically atimed dry plasma etch utilizing CF₄/HBr, and typically extends openings20 to less than or equal to about 500 Angstroms into substrate 12. Apurpose of the trench initiation etch can be to clean an exposed surfaceof silicon substrate 12 within openings 20 (i.e., to remove defects andpolymer material) prior to final trenching into substrate 12. Anotherpurpose of the trench initiation etch can be to form polymer overexposed sidewall edges 15 and 17 of oxide layer 14 and nitride layer 16,respectively. Such polymer can alleviate erosion of sidewall edges 15and 17 during subsequent etching of substrate 12.

Referring to FIG. 4, a third etch is conducted to extend openings 20further into substrate 12 and thereby form trenches within substrate 12.Extended openings 20 comprise a periphery 22 defined by substrate 12.The third etch typically utilizes an etchant consisting entirely of HBr,and is typically a timed etch. The timing of the etch is adjusted toform trenches within substrate 12 to a desired depth. For instance, ifopenings 20 are to be shallow trenches, the third etch will be timed toextend openings 20 to a depth of less than or equal to about one micron.

Referring to FIG. 5, photoresist layer 18 (FIG. 4) is removed and afirst oxide layer 24 is thermally grown within openings 20 and along theperiphery 22 (FIG. 4) defined by silicon substrate 12. The growth ofoxide layer 24 can form small bird's beak regions 26 underlying sidewalledges 17 of nitride layer 16.

Referring to FIG. 6, a high density plasma oxide 28 is formed to fillopenings 20 (FIG. 5) and overlie nitride layer 16. High density plasmaoxide 28 merges with oxide layer 24 (FIG. 5) to form oxide plugs 30within openings 20 (FIG. 5). Oxide plugs 30 have laterally outermostperipheries 33 within openings 20.

Referring to FIG. 7, wafer fragment 10 is subjected to planarization(such as, for example, chemical-mechanical polishing) to planarize anupper surface of oxide plugs 30. The planarization stops at an uppersurface of nitride layer 16.

Referring to FIG. 8, nitride layer 16 is removed to expose pad oxidelayer 14 between oxide plugs 30.

Referring to FIG. 9, pad oxide layer (FIG. 8) is removed. The removal ofthe pad oxide layer leaves dips 32 at edges of oxide plugs 30.

Referring to FIG. 10, a sacrificial oxide layer 34 is grown oversubstrate 12 and between oxide plugs 30.

Referring to FIG. 11, sacrificial oxide layer 34 (FIG. 10) is removed.Formation and removal of sacrificial oxide layer 34 can be utilized toclean a surface of substrate 12 between oxide plugs 30. As such surfaceof substrate 12 can be ultimately utilized to form an active area of atransistor device, it is desired that the surface be substantially freeof defects. The removal of sacrificial oxide layer 34 can alsoundesirably exacerbate dips 32.

Referring to FIG. 12, a silicon dioxide layer 36 is regrown betweenoxide plugs 30, and a polysilicon layer 38 is formed over oxide plugs 30and oxide layer 36. Polysilicon layer 38 can ultimately be formed into aword line comprising transistor gate regions. Such transistor gateregions can lie between oxide plugs 30. Plugs 30 can then function astrenched isolation regions between transistor devices. Dips 32 canundesirably result in formation of parasitic devices adjacent thetransistor devices and ultimately have an effect of lowering a thresholdvoltage for the transistor devices. Accordingly, it would be desirableto alleviate dips 32. Dips 32 can also interfere with subsequentfabrication processes and, for this reason as well, it would bedesirable to alleviate dips 32.

SUMMARY OF THE INVENTION

In one aspect, the invention encompasses an isolation region formingmethod. An oxide layer is formed over a substrate. A nitride layer isformed over the oxide layer. The nitride layer and oxide layer have apattern of openings extending therethrough to expose portions of theunderlying substrate. The exposed portions of the underlying substrateare etched to form openings extending into the substrate. After etchingthe exposed portions of the substrate, portions of the nitride layer areremoved while leaving some of the nitride layer remaining over thesubstrate. After removing portions of the nitride layer, oxide is formedwithin the openings in the substrate. The oxide within the openingsforms at least portions of isolation regions.

In another aspect, the invention encompasses another embodimentisolation region forming method. A silicon nitride layer is formed overa substrate. A masking layer is formed over the silicon nitride layer. Apattern of openings is formed to extend through the masking layer and tothe silicon nitride layer. The openings are extended through the siliconnitride layer to the underlying substrate. The silicon nitride layer hasedge regions proximate the openings and has a central region between theedge regions. The openings are extended into the underlying substrate.After extending the openings into the underlying substrate, a thicknessof the silicon nitride layer is reduced at the edge regions to thin theedge regions relative to the central region. Oxide is formed within theopenings that are extended into the substrate. The oxide within theopenings forms at least portions of isolation regions.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

FIG. 1 is a diagrammatic, fragmentary, cross-sectional view of asemiconductor wafer fragment at a preliminary step of a prior artprocessing sequence.

FIG. 2 shows the FIG. 1 wafer fragment at a prior art processing stepsubsequent to that of FIG. 1.

FIG. 3 shows the FIG. 1 wafer fragment at a prior art processing stepsubsequent to that of FIG. 2.

FIG. 4 shows the FIG. 1 wafer fragment at a prior art processing stepsubsequent to that of FIG. 3.

FIG. 5 shows the FIG. 1 wafer fragment at a prior art processing stepsubsequent to that of FIG. 4.

FIG. 6 shows the FIG. 1 wafer fragment at a prior art processing stepsubsequent to that of FIG. 5.

FIG. 7 shows the FIG. 1 wafer fragment at a prior art processing stepsubsequent to that of FIG. 6.

FIG. 8 shows the FIG. 1 wafer fragment at a prior art processing stepsubsequent to that of FIG. 7.

FIG. 9 shows the FIG. 1 wafer fragment at a prior art processing stepsubsequent to that of FIG. 8.

FIG. 10 shows the FIG. 1 wafer fragment at a prior art processing stepsubsequent to that of FIG. 9.

FIG. 11 shows the FIG. 1 wafer fragment at a prior art processing stepsubsequent to that of FIG. 10.

FIG. 12 shows the FIG. 1 wafer fragment at a prior art processing stepsubsequent to that of FIG. 11.

FIG. 13 is a schematic, fragmentary, cross-sectional view of asemiconductor wafer fragment in process according to a first embodimentmethod of the present invention. The processing step illustrated in FIG.13 is subsequent to the prior art processing step shown in FIG. 3.

FIG. 14 shows the FIG. 13 wafer fragment at a processing step subsequentto that of FIG. 13.

FIG. 15 shows the FIG. 13 wafer fragment at a processing step subsequentto that of FIG. 14.

FIG. 16 shows the FIG. 13 wafer fragment at a processing step subsequentto that of FIG. 15.

FIG. 17 is a schematic, fragmentary, cross-sectional view of asemiconductor wafer fragment in process according to a second embodimentmethod of the present invention. The wafer fragment of FIG. 16 is shownat a processing step subsequent to the prior art processing step of FIG.4.

FIG. 18 shows the FIG. 17 wafer fragment at a processing step subsequentto that of FIG. 17.

FIG. 19 shows the FIG. 17 wafer fragment at a processing step subsequentto that of FIG. 18.

FIG. 20 shows the FIG. 17 wafer fragment at a processing step subsequentto that of FIG. 19.

FIG. 21 shows the FIG. 17 wafer fragment at a processing step subsequentto that of FIG. 20.

FIG. 22 is a schematic, fragmentary, cross-sectional view of asemiconductor wafer fragment in process according to a third embodimentmethod of the present invention. The wafer fragment of FIG. 20 is shownat a processing step subsequent to the prior art processing step of FIG.4.

FIG. 23 shows the FIG. 22 wafer fragment at a processing step subsequentto that of FIG. 22.

FIG. 24 shows the FIG. 22 wafer fragment at a processing step subsequentto that of FIG. 23.

FIG. 25 shows the FIG. 22 wafer fragment at a processing step subsequentto that of FIG. 24.

FIG. 26 is a schematic, fragmentary, cross-sectional view of asemiconductor wafer fragment in process according to a fourth embodimentmethod of the present invention. The wafer fragment of FIG. 26 is shownat a processing step subsequent to the prior art processing step of FIG.3.

FIG. 27 shows the FIG. 26 wafer fragment at a processing step subsequentto that of FIG. 26.

FIG. 28 shows the FIG. 26 wafer fragment at a processing step subsequentto that of FIG. 27.

FIG. 29 shows the FIG. 26 wafer fragment at a processing step subsequentto that of FIG. 28.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstituional purposes of the U.S. Patent Laws “to promote the progressof science and useful arts” (Article 1, Section 8).

The present invention encompasses methods which can alleviate the dips32 described above with reference to the prior art processing shown inFIGS. 1-12. A first embodiment of the present invention is describedwith reference to FIGS. 13-16. In describing the first embodiment,similar numbering to that utilized above in describing the prior artprocessing of FIGS. 1-12 will be used, with differences indicated bysuffix “a” or by different numerals.

FIG. 13 illustrates a semiconductor wafer fragment 10 a at a preliminarystage of the first embodiment method. Specifically, wafer fragment 10 ais illustrated at a processing step subsequent to the prior art step ofFIG. 3. Wafer fragment 10 a comprises a semiconductive substrate 12, anoxide layer 14, a nitride layer 16, and a photoresist layer 18. Openings40 extend through oxide layer 14 and nitride layer 16 and into substrate12. Oxide layer 14 and nitride layer 16 ultimately function as maskinglayers during formation of an isolation region, and so can be referredto as a first masking layer 14 and a second masking layer 16.

The formation of openings 40 can be initiated by processing identical tothat described above with reference to prior art FIG. 3. Specifically,openings 20 (FIG. 3) are formed by transferring a pattern fromphotoresist layer 18 through first and second masking layers 14 and 16.Openings 20 (FIG. 3) are then extended into openings 40 by etchingphotoresist layer 18. Such etching reduces a horizontal width ofphotoresist layer 18 and thereby exposes portions of underlying secondmasking layer 16. The etch of photoresist layer 18 can comprise, forexample, a dry etch utilizing a mixture of an oxygen-containing materialand He. The oxygen-containing material can comprise, for example, O₂present in a concentration greater than or equal to about 10%.Alternatively, the etch can be a dry etch utilizing 100% O₂. The etchwill generally remove photoresist faster with higher concentrations ofO₂ utilized in the etch than with lower concentrations of O₂. Inembodiments in which masking layers 14 and 16 comprise oxide andnitride, respectively, the above-described etch conditions can alsoremove polymer from exposed portions of nitride layer 16 and oxide layer14. Such polymer is described in the “Background” section of thisdisclosure with reference to FIG. 3, and is described as protectingnitride sidewalls 17 and oxide sidewalls 15 during a silicon etchdescribed with reference to FIG. 4. Accordingly, removal of such polymerlayer can increase susceptibility of layers 14 and 16 to a subsequentsilicon etch.

Referring to FIG. 14, wafer fragment 10 a is subjected to a siliconetch, such as, for example, the HBr etch described above with referenceto FIG. 4. Such etch extends openings 40 into substrate 12 and alsoremoves exposed portions of nitride layer 16 and oxide layer 14.Accordingly, the etch moves a furthest lateral periphery of the secondmasking layer (defined by sidewalls 17) outward from the opening withoutreducing a thickness of the second masking layer. After the etching,openings 40 comprise a step 42 (corresponding to rounded corners) belowoxide layer 14. Step 42 defines a region where a wider upper portion ofan opening 40 joins to a narrower lower portion of the opening 40.

Referring to FIG. 15, photoresist layer 18 (FIG. 14) is removed and anoxide layer 44 is thermally formed within openings 40 by, for example, aprocess analogous to that discussed above with reference to the priorart wafer fragment of FIG. 5. An exemplary process for thermally growingoxide is to expose wafer fragment 10 a to a mixture of Ar and O₂, at atemperature of about 1050° C. and a pressure of about 1 atmosphere, fora time of from about 10 to about 15 minutes. After the formation ofoxide layer 44, subsequent processing analogous to that discussed abovewith reference to FIGS. 6-12 can then be conducted to form isolationregions within openings 40.

FIG. 16 illustrates wafer fragment 10 a after such subsequentprocessing. Specifically, FIG. 16 shows wafer fragment 10 a afterisolation regions 46 have been formed within openings 40 (FIG. 15), andafter a polysilicon layer 38 is provided over the isolation regions. Asshown, steps 42 define an outer lateral periphery of isolation regions46. Such outer periphery is further outward than an outward periphery 33of isolation regions 30 of FIG. 12. Such has resulted in the alleviation(shown as elimination) of dips 32 (FIG. 12) of the prior art isolationregions.

A second embodiment method of the present invention is described withreference to FIGS. 17-21. In describing the second embodiment, similarnumbering to that utilized in describing the prior art of FIGS. 1-12will be used, with differences indicated by the suffix “b” or bydifferent numerals.

Referring to FIG. 17, a wafer fragment 10 b is illustrated at apreliminary processing step of the second embodiment method.Specifically, wafer fragment 10 b is illustrated at a processing stepsubsequent to the prior art step illustrated in FIG. 4, with photoresistlayer 18 (FIG. 4) having been removed. Wafer fragment 10 b comprisessilicon substrate 12, oxide layer 14, and nitride layer 16, with layers14 and 16 alternatively being referred to as first and second maskinglayers, respectively. Openings 50 extend through nitride layer 16 andoxide layer 14, and into substrate 12. Openings 50 can be formed inaccordance with the methods described above with reference to FIG. 4 forforming openings 20.

Referring to FIG. 18, wafer fragment 10 b is exposed to a wet etch whichisotropically etches nitride layer 16 relative to oxide layer 14 andsilicon substrate 12. Such etch can comprise, for example, a dip ofwafer fragment 10 b into phosphoric acid (H₃PO₄) at a temperature of150° C. and ambient pressure. Such dip has been found to consistentlyetch silicon nitride at a rate of about 55 Angstroms per minute. Theetch reduces a thickness of nitride layer 16 and at the same time movessidewalls 17 of nitride layer 16 outwardly from openings 50 to widen atop portion of openings 50. The nitride etch thus results in theformation of steps 52 within openings 50. Steps 52 define a locationwhere a wider upper portion of openings 50 joins a narrower lowerportion of openings 50. Steps 52 have an upper surface comprisingsilicon oxide of oxide layer 14.

Preferably, nitride layer 16 has a thickness of at least about 600Angstroms over substrate 12 after the above-discussed phosphoric acidetch. If remaining nitride layer 16 is less than 600 Angstroms thick, itis found to be less capable of functioning as an etch stop forsubsequent etching (such as the etching described with reference toprior art FIG. 7). Typically, from about 50 Angstroms to about 250Angstroms of nitride is removed from nitride layer 16 during thephosphoric acid etch.

Referring to FIG. 19, substrate 10 b is exposed to a hydrofluoric acidetchant to selectively remove portions of pad oxide layer 14. Theremoval of portions of pad oxide 14 drops steps 52 to an upper surfaceof substrate 12. In some applications, it can be equally preferable toforego such pad oxide etch and proceed directly to the oxidationdescribed with reference to FIG. 20.

Referring to FIG. 20, wafer fragment 10 b is exposed to oxidizingconditions which form an oxide layer 56 within openings 50. Oxide layer56 overlies steps 52.

Referring to FIG. 21, wafer fragment 10 b is exposed to subsequentprocessing analogous to the prior art processing described above withreference to FIGS. 6-12 to form isolation regions 58 and a polysiliconlayer 38 overlying isolation regions 58. As shown, steps 52 define anouter lateral periphery of isolation regions 58. Such outer periphery isfurther outward than an outer periphery 33 of isolation regions 30 ofFIG. 12. Such has resulted in the alleviation (shown as elimination) ofdips 32 (FIG. 12) of the prior art isolation regions.

A third embodiment of the invention is described with reference to FIGS.22-25. In describing the third embodiment, similar numbering to thatutilized above in describing the first two embodiments will be used,with differences indicated by the suffix “c” or by different numerals.

Referring to FIG. 22, a wafer fragment 10 c is shown at a preliminarystage of the third embodiment processing. Wafer fragment 10 c is shownat a processing step subsequent to that of FIG. 4, with a photoresistlayer 18 (FIG. 4) having been removed. Wafer fragment 10 c comprises asemiconductor substrate 12, a pad oxide layer 14, and a silicon nitridelayer 16, with layers 14 and 16 alternatively being referred to as firstand second masking layers, respectively. Openings 60 extend throughlayers 16 and 14, and into substrate 12.

Referring to FIG. 23, nitride layer 16 is subjected to a facet etch toreduce a thickness of portions of nitride layer 16 proximate edges 17.The facet etching can comprise, for example, a plasma etch utilizingargon in combination with a fluorine-containing compound (e.g., CH₂F₂).Preferably, the mixture of argon and fluorine-containing gas comprisesless than or equal to about 5% fluorine-containing gas (by volume). Anexemplary pressure condition of the facet-etching is from about 2 mTorrto about 20 mTorr.

Either before or after the facet etching, wafer fragment 10 c issubjected to HF etching to remove portions of oxide layer 14 from underedges 17 of nitride layer 16. The removal of the portions of oxide layer14 leaves exposed corners 61 of an upper surface of silicon substrate12.

Referring to FIG. 24, wafer fragment 10 c is subjected to oxidationwhich forms an oxide layer 62 within openings 60. The facet etching ofnitride layer 16 prior to thermal oxidation results in rounding ofcorners 61 due to lifting of the edges of faceted nitride layer 16. Therounding of corners 61 is more pronounced than rounding of any analogouscorners in the prior art processing described above with reference toFIG. 5.

Subsequent processing analogous to the prior art processing of FIGS.6-12 results in a structure shown in FIG. 25 comprising isolationregions 64 and a polysilicon layer 66 overlying isolation region 64. Itis noted that the faceted edges of nitride layer 16 can lead tooverhanging oxide ledges (not shown) of the isolation oxide formedduring application of the subsequent processing of FIGS. 6-12 to thestructure of FIG. 24. If such overhanging oxide ledges are formed, theyare preferably removed prior to formation polysilicon layer 66. Suchoverhanging oxide ledges can be removed by, for example,chemical-mechanical polishing of the isolation oxide.

FIG. 25 illustrates that rounded corners 61 have alleviated formation ofdips 32 (FIG. 12) of the prior art.

A fourth embodiment of the present invention is described with referenceto FIGS. 26-29. In describing the fourth embodiment, similar numberingto that utilized above in describing the first three embodiments will beused, with differences indicated by the suffix “d”or by differentnumerals.

Referring to FIG. 26, a wafer fragment 10 d is shown at a preliminarystage of the fourth embodiment method. Specifically, wafer fragment 10 dis shown at a processing step subsequent to the prior art processingstep of FIG. 3. Wafer fragment 10 d comprises a substrate 12, a padoxide layer 14 and a nitride layer 16, with layers 14 and 16alternatively being referred to as first and second masking layers,respectively. Additionally, substrate 12 comprises a photoresist layer18 and openings 70 extending through layers 18, 16 and 14, and intosubstrate 12. Openings 70 can be formed by, for example, prior artmethods described above for forming openings 20 of FIG. 3. Afterformation of openings 70, photoresist layer 18 is etched back by, forexample, a dry etch utilizing an oxygen-containing material, such as theetch described above with reference to FIG. 13. Such etch exposesportions of nitride layer 16, while leaving other portions covered byphotoresist 18.

Referring to FIG. 27, the exposed portions of nitride layer 16 areexposed to additional etching conditions, such as, for example, aphosphoric acid etch as described above with reference to FIG. 18, toreduce a thickness of the exposed portions of the nitride layer.Specifically, the original nitride layer had a thickness of “A” (whichremains the thickness of an unetched central region of the nitridelayer), and the etched portion of the nitride layer (the edge regions)has a thickness of “B”. Preferably, “B” is about one-half “A”. Theetching does not move the furthest lateral periphery of nitride layer 16(defined by sidewall 17) outward from openings 70.

Referring to FIG. 28, wafer fragment 10 d is exposed to oxidizingconditions which grow an oxide layer 72 within openings 70. The thinnedregions of nitride layer 16 are relatively easily lifted by the growingoxide such that “birds beaks” are formed under the thinned regions ofnitride layer 16. The birds beaks are extended relative to any birdsbeaks formed during the prior art processing described above withreference to FIG. 5. Photoresist layer 18 is removed prior to theexposure of wafer fragment 10 d to oxidizing conditions.

Referring to FIG. 29, wafer fragment 10 d is exposed to subsequentprocessing conditions analogous to the prior art processing describedabove with reference to FIGS. 6-12 to form isolation regions 74 andpolysilicon layer 38 overlying isolation regions 34. It is noted thatthe reduced-thickness edges of nitride layer 16 can lead to overhangingoxide ledges (not shown) of the isolation oxide formed duringapplication of the subsequent processing of FIGS. 6-12 to the structureof FIG. 27. If such overhanging oxide ledges are formed, they arepreferably removed prior to formation polysilicon layer 38. Suchoverhanging oxide ledges can be removed by, for example,chemical-mechanical polishing of the isolation oxide.

The processing of FIGS. 26-29 alleviates the prior art dips 32 describedabove in the “Background” section (shown as elimination of dips 32).

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

What is claimed is:
 1. An isolation region forming method comprising thesequential steps of: forming an oxide layer over a substrate; forming asilicon nitride layer over the oxide layer, the silicon nitride layerand oxide layer having a pattern of openings extending therethrough toexpose portions of the underlying substrate; etching the exposedportions of the underlying substrate to form openings extending into thesubstrate, each opening defining a periphery, and the etching forminglateral walls of the oxide and silicon nitride layers with the lateralwalls defining a portion of the periphery of the openings; after etchingthe exposed portions of the underlying substrate, wet etching thesilicon nitride layer to remove portions of the silicon nitride layerwhile leaving other portions of the silicon nitride layer over thesubstrate, the wet etching removing the silicon nitride layer at a rateof about 55 Angstroms per minute; after etching the exposed portions ofthe underlying substrate, removing portions of the oxide layer to extendthe lateral walls of the oxide layer outwardly from the periphery of theopenings and to undercut the silicon nitride layer; and after the wetetching and removing the portions of the oxide layer, and while leavingthe other portions of the silicon nitride layer over the substrate,forming oxide within the openings in the substrate, the oxide within theopenings forming at least portions of isolation regions.
 2. The methodof claim 1 wherein said other portions of the silicon nitride layer havea thickness of at least about 600Å after the wet etching.
 3. The methodof claim 1 wherein the wet etching comprises exposing the siliconnitride layer to phosphoric acid.
 4. An isolation region forming methodcomprising performing the following steps in sequential order: forming asilicon oxide layer over a substrate; forming a silicon nitride layerover the silicon oxide layer, the silicon nitride layer and siliconoxide layer having a pattern of openings extending therethrough toexpose portions of the underlying substrate; etching the exposedportions of the underlying substrate to form openings extending into thesubstrate, each opening defining a periphery, and the etching forminglateral walls of the silicon oxide and silicon nitride layers whereinthe lateral walls define portions of the periphery of the openings;after etching the exposed portions of the underlying substrate, removingportions of the silicon nitride layer to extend the lateral walls of thesilicon nitride layer outwardly from the periphery of the openings;after etching the exposed portions of the underlying substrate, removingportions of the silicon oxide layer to extend the lateral walls of thesilicon oxide layer outwardly from the periphery of the openings and toundercut the silicon nitride layer; and after removing portions of thesilicon nitride and silicon oxide layers, and while leaving portions ofthe silicon nitride layer over the substrate, forming oxide within theopenings in the substrate, the oxide within the openings forming atleast portions of isolation regions.
 5. The method of claim 4 whereinremoving portions of the silicon oxide layer defines a first dimensionfrom the periphery of the openings to the lateral walls of the siliconoxide layer and wherein removing portions of the silicon nitride layerdefines a second dimension from the periphery of the openings to thelateral walls of the silicon nitride layer, and wherein the firstdimension is greater than the second dimension.
 6. An isolation regionforming method comprising performing the following steps in sequentialorder: forming a silicon oxide layer over a substrate; forming a siliconnitride layer over the silicon oxide layer, the silicon nitride layerand silicon oxide layer having a pattern of openings extendingtherethrough to expose portions of the underlying substrate; etching theexposed portions of the underlying substrate to form openings extendinginto the substrate, each opening defining a periphery, and the etchingforming lateral walls of the silicon oxide and silicon nitride layerswherein the lateral walls define portions of the periphery of theopenings; removing portions of the silicon nitride layer to extend thelateral walls of the silicon nitride layer outwardly from the peripheryof the openings, the removing portions of the silicon nitride layercomprising a removal rate of about 55 Angstroms per minute; afteretching the exposed portions of the underlying substrate, removingportions of the silicon oxide layer from beneath the silicon nitridelayer; and after removing portions of the silicon oxide layer, formingoxide within the openings in the substrate, the oxide within theopenings forming at least portions of isolation regions.